The present invention relates to data processing systems and, more particularly, to the communications between such systems.
Various interface standards have been developed for interconnecting data processing systems. In the U.S., one of the more common standards is the RS-232 standard published by the Electronics Industry Association (EIA). Many computer devices are manufactured with Input/Output (I/O) ports or interfaces that conform to the RS-232 standard.
There are some disadvantages to an RS-232 interface. For example, because of the nature of the interface, there are limits to the distance over which communications can be established between two RS-232 devices. For this reason, some computer devices, particularly those designed to be connected to other devices over a distance of more than 15 meters, are designed with interfaces conforming to the RS-422 or RS-485 standards, also published by the Electronic Industry Association.
Since some computer devices have RS-232 interfaces and others have RS-422 or RS-485 interfaces, there is sometimes a need to connect devices having different interfaces, e.g., connecting a device with an RS-232 interface to a device with an RS-422 interface. While converter devices (sometimes referred to as "short haul modems") are commercially available for connecting such devices, they have the disadvantage of requiring an external voltage source in order to operate the circuitry in the converter device. Requiring such an external voltage, and the attendant need for equipment (such as a power supply), considerably increase the expense associated with using such converter devices.
It has been proposed to interconnect data processing devices using modems or interconnects that are self-powered, i.e., derive operating voltages from the signals on their I/O lines rather than from a separate external power source. For example, in U.S. Pat. No. 4,607,170, issued to Miles X. Wickman, there is shown an interconnection device for interconnecting two devices having RS-232 interfaces. The interconnection device has a diode-capacitor network which provides an operating or supply voltage for the circuitry within the device, with the supply voltage derived from the signals available at the I/O ports of the interconnected equipment. However, the interconnection device shown in Wickman is not useable between devices having different interface standards (e.g., an RS-232 device and an RS-422 device) and further has a disadvantage that the signals available at the I/O port of the interconnected equipment may sometimes not have sufficient signal strength from which to derive an adequate operating voltage.
This is a particular problem when one connects a single device of one standard (e.g., a computer having a RS-232 interface) to a multiplicity of devices having a different standard (e.g., terminals each having an RS-422 interface). The operating voltages or power that can be derived from the signal lines (e.g., at the RS-232 interface) may not be sufficient to generate an operating voltage for driving the signal lines leading to each of the terminals. Furthermore, there are ranges of signal strength that conform to the published standards. For example, under the RS-232 standard, the I/O signal lines are permitted to have voltage ranges as small as -5 V to +5 V, or as large as -12 V to +12 V. While a signal having an amplitude of +12 V may be adequate to derive operating voltages, a signal having an amplitude of only +5 V may not.